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This invention relates to an apparatus for detecting the relevant properties of subterranean formations while drilling wells for oil, natural gas, and geothermal energy. More specifically, the invention relates to the measurement of the linear rebound position, velocity, and acceleration of a down-hole fluid-driven percussive piston which impacts a drill bit, thus penetrating the subterranean formations. Such pistons are referred to in the industry as xe2x80x9cdown-hole hammers,xe2x80x9d examples of which are disclosed in U.S. Pat. Nos. 5,396,965, 5,488,998, and 5,497,839. U.S. Pat. No. 5,396,965 discusses in detail the principles involved in the operation of a down-hole hammer actuated by drilling mud. These patents are incorporated herein. Drilling mud is only one of a number of different fluids used to drive down-hole hammers; any gas or liquid such as air, water, brine, or a foam combination could drive the mechanism upon which this invention is based.
The materials to be penetrated by such a drill vary by the type, depth, and location of the well. Knowledge of the lithology being drilled assists the drilling crew in the selection of drilling parameters and indicates when a xe2x80x9cpay zonexe2x80x9d is near. Thus, it would be very useful to obtain information about the physical characteristics of the formations being drilled. Specifically, it is helpful to know the hardness of the formations, their approximate composition, and whether or not they are part of a subterranean fracture. This would allow the surface team to steer the bit towards places where the type of energy being sought can be efficiently acquired. In the drilling process, it is also advantageous to optimize the drilling rate to minimize labor and tool replacement costs. The optimum set of conditions such as frequency of hammer impact, force of impact, presence of pulsed jet action, and rotational speed of the drill bit, varies by the hardness, depth, and composition of the formations being drilled. For example, impact-resistant diamond cutters that have a long life in medium hardness rock will tend to decompose and wear away under the high temperatures generated by drilling through hard, abrasive formations. If real-time data on the characteristics of the subterranean formations being penetrated could be acquired, a control system could be implemented to adapt the drilling conditions to the type of rock encountered.
When two objects collide, the energy with which they rebound depends upon the elasticity and hardness of both. Thus, an analysis of the rebound characteristics (position, velocity, and acceleration over time) of the hammer will also reveal the hardness and general makeup of the formations in contact with the bit. The harsh environment in which the hammer operates makes conventional measurement methods impractical; potentiometers, interferometers, and other instruments that measure displacement will not function well in the presence of high-pressure abrasive fluids, vibrations, and impact forces. There is a need for some novel method of discovering the hammer""s position, velocity, and acceleration during the short period of time after it strikes the drill bit.
Additional utility would attend such a method if it could measure the position, velocity, or acceleration of the hammer at other points in its reciprocating motion as well. For example, simple knowledge of the impact velocity or frequency of the hammer may be used to help detect wear or malfunction of the hammer. As a second example, knowledge of the position of the hammer may enable the use of more flexible electromechanical valves to control hammer motion. In typical hammer mechanisms, such as those described in the patents above, the only variables that can be altered during drilling are the pressure or flow rate of fluid entering the drill string. This allows the drilling team to control only the frequency and force of impact, both of which must simultaneously increase or decrease. Optimization of the drilling process requires more control over the motion of the hammer, some of which can be attained through the use of computer-controlled valves. These would allow the drilling crew to dynamically modify the stroke of the hammer, thus, for example, increasing the hammer frequency while decreasing the force of imp act, etc. This type of control system would need data describing the hammer""s displacement over its full range of motion.
Several different types of transducers exist; they are based upon principles such as variable resistance, optical interference, acoustic rebound, piezoelectric excitation, and magnetic flux variance. The following are examples of some that could be configured to measure the motion of the piston.
One means of optical measurement is the interferometer. It functions by focusing a beam of coherent light through a beam splitter. One part of the beam bounces off of a stationary mirror while the other bounces off of a moveable mirror; when the two returning beams are simultaneously visible, differences in the observed wavelengths indicate the displacement of the moveable mirror. In hammer machines that run on transparent fluids such as air, this can be used to determine the location of the piston over time.
Acoustic rebound transducers utilize sonic or ultrasonic waves and measure the speed of a passing object by utilizing such phenomena as the xe2x80x9cDoppler effect.xe2x80x9d
Variable resistance transducers include potentiometers, which measure the displacement of an electrical contact along a coil of wire. The wire of the coils is of a known resistivity; when the contact closes the circuit with a known voltage source by touching one of the coils, the resulting output voltage is proportional to the length of wire the current must travel through. Thus, the voltage is a measure of the relative displacement of the contact and the coil.
Piezoelectric transducers function based on the unique tendency of some materials, such as single crystal quartz, to develop a charge when subjected to a mechanical strain. The charge generated is proportional to the force on the crystal; thus, the piezoelectric load cell can be used to measure force. This, in turn, yields a measurement of the acceleration when the load cell is attached to a moving object such as the piston; the weight of the cell will press on the crystal to produce a measurable charge in proportion to the magnitude of the acceleration. Since they measure changes in force, piezoelectric crystals can also readily be configured to measure pressure changes in fluids. Such a pressure transducer mounted in the fluid cavity above the piston or likewise on the drill string closer to the point of impact would yield data that generally describes the motion of the hammer, as derived from the cyclical fluid pressure variations.
There are also other types of transducers that operate based on measurement of changes in magnetic flux. Linear variable displacement transducers, or LVDT""s, have one coil wrapped around a magnetically permeable core. When the core moves between two other concentric coils, the ac voltage through the first coil will excite a voltage output in the other two in proportion to the core""s proximity to them. Thus, the LVDT measures the location, or displacement, of the core.
When the piston strikes the impact mass, there are two components to its motion: the transient response and the steady response. The transient response is the portion of the waveform induced as a direct result of the impact; its amplitude upon impact is significant but drops to zero before the end of the cycle. The steady response is the normal, near-sinusoidal waveform of the piston""s motion resulting from the fluid pressures that actuate it. The transient response yields information regarding the impact and consequently the physical makeup of the formations being impacted. The steady response describes the piston""s general motion and therefore provides data that can be used to deduce the piston""s frequency, stroke length, and impact force.
For the analysis of subterranean features as well as diagnostic testing of the hammer""s operation, the displacement, velocity, and acceleration of the piston are all useful quantities. If one is known, the other two can be determined from it by integration or differentiation. However, since the piston does not always strike the impact mass or reach the same point at the top of its stroke, there may be a need for a position datum if displacement is not the variable being directly measured. In other words, it might be necessary to know at which point in time the piston reaches a certain position once each cycle because inaccuracies in the integration over time may build up and yield an inexact measurement of the position of the piston. The sensor could be a simplified version of any of the displacement transducers discussed above, as it only needs to provide a simple signal to indicate that the piston has reached the predetermined position.
For the invention, the magnetic flux-based transducer was chosen as the most viable for down-hole applications. Lateral vibrations in the piston""s movement, abrasive down-hole conditions, and high velocities make it difficult to use any transducer in which the moving and stationary parts must be in contact with each other, such as the potentiometer. It is similarly impractical to extend any wires from the piston to a stationary part of the drill string because the piston""s motion will tend to break the wires in fatigue while the abrasive effects of the drilling fluid will rapidly wear away exposed electrical conductors. For these reasons, it is desirable to use a transducer in which the only communication between moving and stationary parts does not require contact between the piston and the drill string. A magnetic coupling accomplishes this criterion and provides a particular advantage for down-hole applications, since such a coupling may operate in typically opaque drilling mud. Although either permanent or electrically activated magnets can be used, permanent magnets are preferable for mounting in a hammer piston because they reduce the number of electrical contacts required.
Yet further functionality of the above-mentioned measurement method is apparent when one considers that an electrical signal generated by this method may be useable as an electrical power source. Data acquisition, data transmission, and control systems, as described above, require a steady source of power down-hole. Due to the time and expense required to retrieve the drill from the borehole, it is critical to find a power system that will operate for as long as possible without the need for maintenance or replacement. A down-hole power system should be designed to operate for at least 100 hours.
Several methods of providing electricity down-hole have been tested with limited success. New lithium technology currently being implemented in batteries cannot provide a long enough life to be useful for powering complex systems down-hole. The abrasive environment makes turbines and other bladed rotary generators particularly short lived. The motion of the down-hole hammer, however, may be used to provide the needed means of down-hole electricity generation.
The present invention is a method of measuring the motion characteristics of the hammer through the use of a transducer mounted on the piston, drill bit, or drill string. A transducer is a device that converts one form of energy to another. Thus, in this invention, a portion of the energy resident in the hammer is converted into electrical energy. The present invention thus becomes not only a motion sensor, but also an electrical generator.
The preferred embodiment of the transducer consists of a series of coils and magnetic flanges mounted inside the hammer. In this embodiment, the transducer will provide valuable information on the transient and steady motion of the hammer in the form of an electric signal strong enough to constitute a power source.
The present invention constitutes a method for satisfying three functional needs in implementing down-hole control, measurement, and telemetry systems. This device provides a means of measuring the hammer""s impact characteristics, a means of determining its general displacement profile over time, and a means of generating electrical power for use in down-hole systems.
Combining existing technologies solves these problems. First of all, fluid-driven hammers, as shown in U.S. Pat. Nos. 5,396,965, 5,488,998, and 5,497,839 are used to convert hydraulic pressure to linear kinetic energy. Linear alternators, such as those described in U.S. Pat. Nos. 4,454,426, 4,602,174, 5,180,939, and 5,389,844 convert linear kinetic energy to electrical energy. These patents are incorporated herein. The amplitude of the electric waveform produced is in proportion to the velocity of the linear reciprocator.
The basic principles are as follows. When a magnetic field passing through a coil changes in strength or orientation, a voltage is induced in proportion to the change in the field divided by the time required for the change to occur. This principle is commonly used to convert between mechanical and electrical forms of energy. The relative motions of the magnets and the coils can be rotational or linear. Linear reciprocating elements have been used as components for linear motors and alternators, such as those disclosed in U.S. Pat. Nos. 4,454,426 and 4,602,174, as well as measurement devices such as linear variable displacement transducers (LVDT""s). A coil and magnet system is used to measure the location of a compressor piston with respect to the cylinder head in U.S. Pat. No. 5,342,176, incorporated herein by this reference.
The preferred embodiment of this invention utilizes permanent magnets mounted on a stationary member in the hammer housing. Coils are also mounted on a stationary member and the motion of the hammer (a mass of magnetic material) past the permanent magnets causes a changing flux field. This changing flux field induces a current in the coils, which can be measured and used to feed electrical devices. In a second embodiment of this invention, the magnets may be mounted on the hammer, which reciprocate with respect to coils mounted on a stationary member in the hammer housing. Similarly, the measurement device can be mounted on the drill bit, which will display a similar rebound characteristic. The hammer""s position and acceleration can also be obtained by integration and differentiation, respectively. This yields the desired information concerning the rebound and general motion of the hammer.
The voltage induced by such a configuration produces an alternating current waveform. If the magnetic field is strong and there are a large number of coils, the amplitude of the waveform will be large enough to constitute a signal useful as a power source. See U.S. Pat. No. 4,491,738, incorporated herein, for an example of a machine that generates small amounts of electricity down-hole independent of a hammer.